A hybrid-type construction machine, which operates using both a power of an internal combustion engine and a power of an electric motor, has been developed and used. As a hybrid-type construction machine, there is known an example which takes a drive mode of a so-called parallel system.
In the parallel system drive mode, a hydraulic pump and a drive machine, which has a generator function and a motor function, are connected in parallel to an internal combustion engine (engine) as a common power source. Hydraulic actuators are driven by the hydraulic pump, and a power accumulation device is charged according to the generator function of the drive machine. The drive machine is operated as an electric motor by being supplied with an electric power from the power accumulation device in order to assist the engine. It should be noted that, as a drive machine, there is a case where a combined machine (generator and motor) having both a generator function and a motor function is used, and a case where a generator and a motor, which are separated from each other, are used in combination.
In the above-mentioned hybrid-type construction machine, the hydraulic pump is driven by, for example, an internal combustion engine (engine), such as a diesel engine, a gasoline engine, etc. Because a maximum output of an internal combustion engine is determined by a number of revolutions of the internal combustion engine at that time, an upper limit value corresponding to a number of revolutions is set to the output of the internal combustion engine. Accordingly, if a power requested by the hydraulic pump to the engine exceeds the upper limit value determined by the number of revolutions of the internal combustion engine at that time, it is not possible to output the requested power.
Therefore, in a hybrid-type construction machine, if a power requested by the hydraulic pump to the engine exceeds the upper limit value, a drive machine is used as a motor to assist an engine to compensate for an output corresponding to a portion exceeding the upper limit value. As mentioned above, the hybrid-type construction machine has an advantage in that a rated output of an engine can be reduced because there is an assist by an electric motor.
For example, it is suggested to use an engine having a small rated output and obtain a necessary hydraulic output by assisting the engine by an electric motor when a large hydraulic output is required (for example, refer to Japanese Laid-Open Patent Application No. 2004-11502).
However, if an assist by the motor is continuously performed, or frequently performed, it becomes difficult to maintain a state of charge of a battery for driving the electric motor. Thus, a control is performed so as to use the engine effectively as much as possible to prevent an excessive use of battery power by setting a power of the hydraulic pump to be lower than a rated output of the engine. Alternatively, an assist by the motor is suppressed by controlling the output of the hydraulic pump to be reduced in response to the state of charge of the battery.